Many electronic images such as news and sporting events are taken under uncontrolled lighting conditions. Thus image dynamic range and contrast may be less than aesthetically pleasing. On the other hand, medical images such as fluoroscopic videos may contain such poor contrast and dynamic range so as to limit their value. These and other electronic imaging conditions need to be addressed in a timely and cost effective manner.
A wide variety of techniques and systems have been devised for the processing of electronic image signals with less than ideal dynamic range and/or contrast. A large number of systems are based first upon the analysis of the image using a histogram. Then based upon this analysis the image is modified under operator control. Some techniques consist primarily of simple gray scale modification. On the other hand, some techniques are more sophisticated such as the system set forth in U.S. Pat. No. 5,012,333 to Lee et al. Most of these techniques depend upon a relatively slow computer to perform signal processing. In general these techniques are effective in image improvement, but they can be relatively time consuming especially in the derivation of the histogram. When many frames of electronic images such as live sports, news events, or fluoroscopic images are required to be processed at real time or close to real time most techniques are too time consuming. Faster computers could be employed for signal processing, but this usually increases the cost substantially.
The text Digital Signal Processing by Alan V. Oppenheim and Ronald W. Schafer, Prentice-Hall, 1975 in the chapter titled Homomorphic Signal Processing teaches that some classes of images can be improved with signal processing without extensive analysis of a histogram. The author states that observed images are formed by a multiplying of pattern illumination by pattern reflectance to produce the brightness image. The author also, states that these components of the brightness image can be separated as a function of frequency content. That is to say, in general the illumination component is in a different frequency range than the reflectance component. The author concludes that in general within an electronic image illumination is a low frequency signal and reflectance is a high frequency signal.
The author's signal processing gave image improvement, but addresses only a very limited number of image problems. Therefore, if one could implement a system that could be used to improve a larger variety of image dynamic range and contrast type problems in a timely manner using adjustable hardware, this would be desirable.